A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. In particular, the fuel cell has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
A typical fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a cathode, an anode and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrode-assembly (MEA). The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of generating a quantity of electricity sufficient to power a vehicle.
During periods of non-operation, a quantity of air accumulates in the anodes of the fuel cell stack. Upon start-up of the fuel cell stack, hydrogen is supplied to the anodes. The hydrogen contacts the air and creates a “hydrogen-air front” that passes over the anodes. The hydrogen-air front is known to degrade fuel cell performance. In particular, the presence of both hydrogen and air on the anode results in a localized short between a portion of the anode that sees hydrogen and a portion of the anode that sees air. The localized short causes a reversal of current flow and increases the cathode interfacial potential, resulting in a rapid corrosion of the fuel cell carbon substrates and catalyst supports. The rate of carbon corrosion has been found to be proportional to a time that the hydrogen-air front exists and a magnitude of the localized voltage at the hydrogen-air front.
It is known in the art to rapidly purge the anodes of the accumulated air with hydrogen and minimize the time that the hydrogen-air front exists on the anodes. The purge is often designed to substantially and evenly fill the anode inlet header with hydrogen without exhausting an excess of hydrogen from the fuel cell system. An illustrative purge method is disclosed in applicant's co-pending U.S. application Ser. No. 11/762,845, incorporated herein by reference in its entirety. Typically, a time required to purge the anodes is calculated in advance, and based on the volume of the fuel cell stack and the flow rate of the hydrogen. However, the quantity of air that has accumulated on the anodes varies with different shut-down periods and conditions. Additionally, variations in pressure, pressure measurements, flow rates, flow control and composition of the gases on the anodes after shut-down periods may vary widely. Therefore, the time required to push the accumulated air from the anodes, as well as the volume and flow rate of hydrogen for purging the anodes, is generally not optimized. As the optimal end point of the purge is often difficult to predict, systems known in the art have been unable to purge the anodes with hydrogen without exhausting an undesirable quantity of hydrogen to the atmosphere.
Additionally, known systems have also employed a dead-short circuit during start-up of the fuel cell stack. In dead-short systems, a circuit with a shorting resistor, for example, is used to minimize the localized voltage during start-up of the fuel cell stack. Resistance to carbon corrosion during start-up of the fuel cell stack is thereby optimized. In order for the dead-short system to work properly, however, each fuel cell in the fuel cell stack must have substantially equal quantities of hydrogen for the duration of the dead-short. A fuel cell that is deficient in hydrogen may experience undesirable, localized “hot-spots” if subjected to the dead-short.
There is a continuing need for a fuel cell system and a method that provide a rapid and reliable start-up. It would be desirable to develop a fuel cell system and a method for facilitating a variable anode flow rate during a start-up of the fuel cell system, wherein the fuel cell system and the method minimize an anode fill time, while also minimizing degradation of the fuel cell system due to a start-up procedure.